Wireless communication method and apparatus for adaptively biasing channel quality indicators to maintain a desired block error rate

ABSTRACT

A method and apparatus for adaptively biasing a channel quality indicator (CQI) used for setting a configuration of communication between a transmitter and a receiver in a wireless communication system. The receiver sends a CQI and positive acknowledgement (ACK)/negative acknowledgement (NACK) messages to the transmitter. The ACK/NACK messages indicate the absence or presence of error, respectively, in a transmitted data packet. The CQI is derived from the signal-to-interference ratio (SIR) and the ACK/NACK messages. The transmitter calculates the block error rate (BLER) of the transmitted data packets based upon the ACK/NACK messages sent from the receiver. The transmitter compares the BLER of the transmitted data packets to a target BLER and biases the CQI based on the comparison in order to achieve the target BLER.

FIELD OF INVENTION

The present invention relates to a wireless communication system. Moreparticularly, the invention relates to adaptively biasing ChannelQuality Indicators (CQIs) in a wireless communication system including abase station and a Wireless Transmit/Receive Unit (WTRU).

BACKGROUND

Adaptive Coding and Modulation (ACM) is an effective technique forproviding link adaptation in both Uplink (UL) and Downlink (DL)communications. ACM is typically accomplished by algorithms workingtogether in both the receiver and the transmitter of a WTRU and/or abase station. The receiver makes an estimate of the channel quality bymeasuring the Signal-To-Interference Ratio (SIR) of one or moretransmissions from the transmitter.

Each CQI may correspond to a particular configuration of radio resourcessuch as code rate and modulation type. After each SIR measurement ismade, the CQI is computed. For example, the SIR may be compared to atable of SIR-CQI pairs and the CQI value that yields the bestperformance, for example in terms of Block Error Rate (BLER) orthroughput, is selected. This is typically performed at the receiver andsent back to the transmitter. The transmitter then selects a radioconfiguration that is no more aggressive than that indicated by thereceived CQI value. In an otherwise unpopulated cell, the transmitterwould simply use a configuration consistent with a channel qualityindicated by the CQI.

Although maintaining the appropriate link adaptation parameters, (e.g.code rate, modulation type, number of codes and power control), is knownto optimize throughput for the radio link, there are cases where theparameters selected by the transmitter will be out-of-sync with channelquality. In essence, the parameter selected by the transmitter will notcorrespond to the CQI judged to optimize the throughput. This case canarise when for example there is insufficient data available for the nexttransmitted packet, the scheduler decides to share resources that wouldotherwise be required, or during conformance testing.

Another problem is that channel quality estimates may be more heavilyquantized than the available set of radio resources, thus the assumedone-to-one mapping between CQI and radio resource configuration is lost.This is particularly true in Third Generation Partnership Project (3GPP)system.

The problem can be illustrated by considering the behavior of anadaptive bias algorithm when the transmitter occasionally uses moreconservative configurations than indicated by the CQI report from thereceiver. When the more conservative configurations are used, errorswill be less frequent and the adaptive algorithm will adapt to requestmore aggressive configuration of resources to maintain the target BLER.When the transmitter returns to normal operation, responding asanticipated to CQI reports, the bias will have been adapted to the wrongvalue and many packets will be lost until reconvergence of thealgorithm.

SUMMARY

The present invention is related to a wireless communication method andapparatus for adaptively biasing a CQI used for setting a configurationof communication between a transmitter and a receiver in a wirelesscommunication system. The apparatus may be a wireless communicationsystem including a receiver and a transmitter, a WTRU, a base stationand/or an integrated circuit (IC).

In the wireless communication system, the receiver sends a CQI and apositive Acknowledgement (ACK)/Negative Acknowledgement (NACK) to thetransmitter. The ACK/NACK messages indicate the absence or presence oferror, respectively, in a transmitted data packet. The CQI is derivedfrom the SIR and the ACK/NACK messages. The transmitter calculates theBLER of the transmitted data packets based upon the ACK/NACK messagessent from the receiver. The transmitter compares the BLER of thetransmitted data packets to a target BLER and biases the CQI based onthe comparison in order to achieve the target BLER.

BRIEF DESCRIPTION OF THE DRAWINGS

A more detailed understanding of the invention may be had from thefollowing description, given by way of example and to be understood inconjunction with the accompanying drawings wherein:

FIG. 1 is a block diagram of a wireless communication system including aWTRU and a base station for performing adaptive CQI biasing according tothe present invention;

FIG. 2 is a block diagram of a wireless communication system wherein theWTRU includes an adaptive SIR bias unit according to the presentinvention;

FIG. 3 is an exemplary block diagram of a loop controller used in thesystems of FIGS. 1 and 2 to control the adaptive CQI biasing accordingto the present invention;

FIG. 4 is a detailed block diagram of an adaptive SIR bias unit used inthe system of FIG. 2; and

FIG. 5 is a flowchart of a process including method steps for managingcommunications using CQI biasing according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereafter, the terminology “WTRU” includes but is not limited to a UserEquipment (UE), mobile station, fixed or mobile subscriber unit, pager,or any other type of device capable of operating in a wirelessenvironment.

When referred to hereafter, the terminology “base station” includes butis not limited to a Node-B, a site controller, an access point or anyother type of interfacing device in a wireless environment.

The present invention is generally applicable to all modes of the 3GPPstandards including Time Division Duplex (TDD), Frequency DivisionDuplex (FDD), Time Division Synchronous Code Division Multiple Access(TDSCDMA) and Code Division Multiple Access 2000 (CDMA 2000) scenarios,but may be applicable to other scenarios as well.

The features of the present invention may be incorporated into an IC orbe configured in a circuit comprising a multitude of interconnectingcomponents.

FIG. 1 is a block diagram of a wireless communication system 10 forperforming adaptive CQI biasing according to the present invention. Thesystem 10 performs adaptive CQI biasing of communication between a basestation 200 and a WTRU 100. The invention will be described withreference to a base station 200 performing the CQI biasing, but itshould be understood by those of skill in the art that it may also beperformed at the WTRU.

Referring to FIG. 1, the WTRU 100 comprises SIR estimator 20, SIR to CQImap unit 30, Cyclic Redundancy Check (CRC) error detector 70 and areceiver 80. A data packet is received and processed by the receiver 80.The SIR estimator 20 estimates a SIR for a reference channel based onthe received data packet. The calculated SIR is mapped by the SIR to CQImap unit 30 to generate a CQI value. The generated CQI value is chosento yield the best performance, such as in terms of BLER or throughputbased upon the fixed SIR to CQI pair table. The CRC error detector 70detects any errors in a received data packet using CRC, and generates anACK or NACK signal as appropriate according to the absence or presenceof errors. An ACK indicates the absence of an error and a NACK indicatesthe presence of an error in the data packet.

Still referring to FIG. 1, the base station 200 comprises a summer 210,a CQI to Transport Format (TF) map unit 220, a transmitter 230, anadaptive correction term generator 250, a loop controller 260, and abias determination unit 270.

The adaptive correction term generator 250 includes an ACK/NACK unit251, a converter 252, a filter 253, a target BLER unit 254, a summer255, and a Proportional Integral Derivative (PID) controller 256.

The occurrence of an error in the data packet sent to the receiver 80 inthe WTRU 100 is identified by the ACK/NACK unit 251 based upon theACK/NACK messages sent from the WTRU 100. The converter 252 maps theACK/NACK messages, (resulting from the computation of the CRC into ‘0s’and ‘1s’ respectively), and calculates the BLER of the current channel.This signal is preferably filtered by the filter 253. It should,however, be understood that the filter 253 is not required. Thebandwidth of the filter 253 provides a mechanism to trade off betweenresponsiveness of the bias term and the smoothness of the bias term. Asimple Infinite Impulse Response (IIR) filter with a slowly decayingexponential impulse response can be used, but other filters may also beused.

The target BLER unit 254 generates a target BLER to achieve the desiredperformance, such as maximizing throughput. Because the CQI value alonecannot ensure the desired performance, a target BLER value is used as areference in order to maximize data throughput. The purpose ofmaintaining a BLER that is close to the target BLER is that it optimizesthe utilization of radio resources and therefore the data throughput ofthe system. The target BLER value may be a static setting or anoperator-determined semi-static setting, (i.e., it may vary but is notexpected to change rapidly).

The target BLER may be adjusted to optimize system throughput based on avariety of criteria. For example, target BLER will generally be sethigher for vehicular speed channels such that the hybrid automaticrepeat request (HARQ) combining may achieve retransmission timediversity gains. Vehicular speed estimation may be performed either atthe WTRU 100 or the base station 200, although the estimate performed bythe WTRU 100 will generally be more accurate. The target BLER may be setbased on the output from the channel-type correction term generator 40.These gains may be larger than the loss due to more aggressive firsttransmission TFs.

The BLER as filtered by the filter 253 is compared with the target BLERvalue by the summer 255 to generate an error signal 287 that representsa deviation of current performance of the WTRU 100 from the targetperformance of the WTRU 100 in terms of BLER.

The PID controller 256 generates the adaptive bias signal 257. The PIDcontroller 256 may also contain higher order linear or non-linearelements. It should be noted that a Proportional (P) controller, or aProportional Integral (PI) or Integral (I) controller, may be usedinstead of the PID controller 256.

The adaptive bias signal 257 generated by the PID controller 256, and abalance value signal 280 generated by the loop controller 260, are inputto the bias determination unit 270. The bias determination unit 270outputs a CQI bias signal 284 to the summer 210. The bias determinationunit 270 provides a means to use one or more default bias terms 282,(e.g., one or more factory or operator set/tuned parameters), in theevent that the CQI values transmitted to the base station 200 are notbeing used as expected. The summer 210 provides a biased CQI value 286to achieve the target BLER, based upon the adaptive bias signal 257.

The biased CQI value 286 is converted to a Transport Format (TF) 225 bythe CQI to TF map unit 220. The TF 225 is a value of a radio resourceconfiguration and includes a block size, a modulation type, a codingrate, and a transmission power. The transmitter 230 then transmits datapackets to the WTRU 100 in accordance with the TF 225. The transmitter230 also provides a New Data Indicator (NDI) 235 to the loop controller260. The NDI is a flag set by the base station 200 and conveyed to theWTRU 100 via a High Speed Synchronization Control Channel (HS-SCCH).

FIG. 2 shows a wireless communication system 15 wherein the WTRU 100also performs adaptive biasing using an adaptive SIR bias unit 60 and asummer 32. The adaptive SIR bias unit 60 generates a SIR bias thatimproves spectral efficiency by efficiently matching the transmittedmodulation format and coding rate to the channel and receiver.

FIG. 3 is an exemplary block diagram of the loop controller 260. Theloop controller 260 controls the overall operation of the adaptivecorrection term generator 250 via a hold signal 276. The BLER estimateat the base station 200 may be a conditional BLER, where the conditionis that the transmission is the first, or an average BLER withoutconditions, (determined by the presence or absence of the NDI signal 235shown in FIG. 3). The NDI signal 235 is obtained by interpretation ofthe transmitted NDI signal generated by the base station 200 and carriedby the HS-SCCH channel. The base station generated NDI toggles betweentwo states whenever new data is being transmitted, and therefore it isnot used directly. Instead, the NDI is derived by checking if the stateof the NDI has changed since the last time it was used for a given HARQprocess. In the case of the WTRU 100, the NDI also needs to be decodedfrom the HS-SCCH channel reception.

The loop controller 260 receives a Channel Quality Indicator Expected(CQIE) value 240. The CQIE value is the CQI value that the WTRU 100expected would be used for a given packet. It is a delayed version ofthe CQI reported.

The loop controller 260 also receives a Generated Transport Format (GTF)242 and a CRC 244. The GTF is determined by the base station 200 as partof a scheduling algorithm. The CRC 244 is computed by the WTRU 100 as anindication of whether or not the transport block was received in error.

FIG. 4 is a detailed block diagram of the adaptive SIR bias unit 60located within the WTRU 100 of the wireless communication system 15. Theadaptive SIR bias unit 60 includes a channel-type correction termgenerator 40, a SIR prediction term generator 50, an adaptive correctionterm generator 250, a loop controller 260, a bias determination unit 270and a summer 52.

As shown in FIG. 4, a subsystem including the adaptive correction termgenerator 250, loop controller 260 and bias determination unit 270generates a CQI bias signal 284 which is input to an external summer 32to bias the SIR that maps into CQI such that the WTRU 100 will achievethe target performance in terms of BLER.

The channel-type correction term generator 40 provides additionalcorrection terms to the input of the summer 52 to bias the SIR estimate.These correction terms include delay spread corrections, (e.g.,orthogonality factors), 41, Doppler spread corrections 42, and othercorrection terms 43, (e.g., battery voltage dependant losses introducedby the WTRU 100, other channel-type corrections, or the like). Since SIRalone does not completely define the quality of the channel, thesecorrections improve system performance. The correction terms may be usedto adjust the SIR bias, the target BLER, or both.

The SIR prediction term generator 50 includes a derivative, (i.e.,D/DT), for linear prediction or other SIR predictive functions unit 55and dead zone, clipping or other non-linear elements 57 which providespredictive terms to the input of the summer 52 to bias the SIR estimate.

The SIR prediction term generator 50 may also be employed to improve thebias by estimating what the SIR will be at a time when the base station200 will transmit the next packet. If the channel type is identified,e.g., as a high speed channel, then an open loop or feed forwardprediction bias term may be introduced. By making use of a predicted SIRrather than a current SIR, CQI reports are more accurate at the timethere are used. The adaptive bias method and apparatus described hereinis compatible with these algorithms.

The channel-type correction term generator 40 and the SIR predictionterm generator 50 may be used alone or in combination with each other orwith the adaptive correction term generator 250 to generate an SIRcorrection. When both the correction terms provided by the channel-typecorrection term generator 40 and the predictive terms provided by theSIR prediction term generator 50 are provided to the summer 52, thesummer 52 outputs a resulting composite correction/predictive terms 34to the external summer 32, where the correction/predictive terms 34 aresummed with the output of the SIR estimator 20 and the CQI bias signal284 to generate a corrected SIR.

The corrected SIR is mapped by the SIR to CQI map unit 30 to generate aCQI value. The CQI value is sent to the base station 200 along with anACK or NACK as appropriate. The base station 200 may then bias the CQIvalue received from the WTRU 100 using the adaptive correction termgenerator 250, loop controller 260 and bias determination unit 270 aspreviously described. As a result, in this embodiment, biasing of theCQI value may be performed in the WTRU 100 and in the base station 200.

In an alternate embodiment, a unit similar to the adaptive SIR bias unit60 may also be incorporated into the base station 200, in lieu of theadaptive correction term generator 250, which would perform CQIpredictions, rather than SIR predictions.

FIG. 4 is an exemplary block diagram of the loop controller 260 whichprovides overall control of adaptive CQI biasing in the WTRU 100 and/orthe base station 200 shown in FIG. 2. The loop controller 260 includes aGTF to Equivalent Channel Quality Indicator (ECQI) map unit 261, firstcomparator 262, second comparator 265, third comparator 267, fourthcomparator 269, a mean filter 263, a balance term generator 264, and alogic control circuit 266. The GTF 242, (i.e., TF), is mapped by aGTF-ECQI map unit 261 to generate an ECQI 268.

The ECQI 268 corresponds to the channel quality associated with the GTF242. The first comparator 262 compares the difference between the ECQI268 and the CQIE 240 to a threshold (T1).

The mean filter 263 produces a measurement proportional to thedifference between the ECQI 268 and provides the measurement to thesecond comparator 265 and the balance term generator 264. The secondcomparator 265 compares the long term average to a threshold (T2).

The controller 266 generates a “hold” signal 276 having a binary valuebased on the respective outputs A, B, of the first and secondcomparators 262, 265, and the value of CQIE 240. If the absolute valueof the difference between the CQIE 240 and the ECQI 268 exceeds T1, orif the long term average of the absolute difference between the CQIE 240and the ECQI 268 exceeds T2, the controller 266 generates the holdsignal 276.

The value of CQIE 240 also may affect the generation of the hold signal276. If the CQIE 240 is determined by the comparator 267 to be less thanor equal to a minimum value of CQI, (T3), and a NACK is sent from theWTRU 100 to the ACK/NACK unit 251 in the base station 200, thecontroller 266 generates the hold signal 276 based on output C of thecomparator 267. If the CQIE 240 is determined by the comparator 269 tobe greater than or equal to a maximum value of CQI, (T4), and an ACK issent from the WTRU 100 to the ACK/NACK unit 251 in the base station 200,the controller 266 generates the hold signal 276 based on the output Dof the comparator 269.

The hold signal 276 is not generated when the CQIE 240 is between T3 andT4, when NDI=TRUE, and the threshold tests performed by comparators 262and 265 are not true.

A balance term generator 264 receives a measurement proportional to thedifference between CQIE 240 and the ECQI 268, and generates a “balancevalue” signal 280. The balance value signal 280 is used by the biasdetermination unit 270 to form a weighted average of the adaptive bias257 and a predetermined, (i.e., in the factory or by an operator),default bias term 282.

The hold signal 276 generated by the loop controller 260 suspends theoperation of the adaptive correction term generator 250 by keeping thefilter 253 and the PID controller 256 from updating. Thus, the biasdetermination unit 270 only produces a default bias term 282.

The bias determination unit 270 receives bias values, which include theadaptive bias 257, having a value AB, from the adaptive correction termgenerator 250, and a default bias 282, having a value DBT. The biasdetermination unit 270 receives a balance signal 280, having a valueBAL, from the loop controller 260, and outputs a CQI bias signal 284having a value BIAS according to Equation (1):BIAS=BAL×DBT+(1−BAL)×AB  Equation (1)

The CQI sent from the WTRU 100 is biased based upon the corrected bias(BIAS) via the summer 210, as described above. The biased CQI is mappedby the CQI to TF map unit 220 to generate a TF 225 used fortransmission. The CQI in 3GPP will span 0-30 in integer steps, but issomewhat arbitrary. The TF 225 is not a number but is, rather, a set ofparameters, e.g., {packet size, number of codes, modulation type,transmit power}.

FIG. 5 is a flowchart of a process 500 including method steps formanaging communications between the WTRU 100 and the base station 200using CQI biasing according to the present invention. The receiver inthe WTRU 100 receives data packets from the base station (step 510). TheWTRU 100 estimates SIR of the received data packets (step 520) and mapsthe SIR onto a corresponding CQI via the SIR to CQI map unit 30 (step530). As explained hereinbefore, if the WTRU 100 is implemented as thealternative embodiment of FIG. 2, the SIR is biased (optional step 525)prior to being mapped into the CQI (optional step 535). While receivingdata packets, the WTRU 100 checks the packets for the presence of errorsand sends an ACK or NACK as appropriate together with the CQI to thebase station (step 540).

The base station biases the CQI received from the WTRU 100 to achieve atarget BLER (step 550). The adaptive correction term generator 250receives the ACK or NACK and calculates the BLER of the data packet. Thenumber of packets used to determine the BLER, may be one or more packetsand is determined by the filter 253. The adaptive correction termgenerator 250 then compares the BLER of the data packet to the targetBLER and produces the adaptive bias signal 257. The adaptive bias 257and a default bias 282 are combined in the bias determination unit 270to produce the CQI bias 284 (BIAS) in accordance with the control of theloop controller 260.

The base station 200 then maps the CQI and the BIAS into a TF (step 560)through the CQI to TF map unit 220. The data packets are thentransmitted according to the TF (step 570). By performing the aboveprocess repeatedly, the target BLER of the data packets sent can beobtained.

Although the features and elements of the present invention aredescribed in the preferred embodiments in particular combinations, eachfeature or element can be used alone (without the other features andelements of the preferred embodiments) or in various combinations withor without other features and elements of the present invention.

While this invention has been particularly shown and described withreference to preferred embodiments, it will be understood by thoseskilled in the art that various changes in forms and details may be madetherein without departing from the scope of the invention as describedabove.

1. A wireless transmit/receive unit (WTRU) comprising: a receiver forreceiving data packets; a signal-to-interference ratio (SIR) estimator,electrically coupled to the receiver, for estimating a SIR of thereceived data packets and producing a SIR estimate; a first summerhaving a first input electrically coupled to an output of the SIRestimator; an adaptive SIR bias unit, electrically coupled to an outputof the receiver and to second and third inputs of the first summer, foradjusting the SIR estimate; and a SIR to channel quality indicator (CQI)map unit, having an input coupled to an output of the first summer, andan output coupled to an input of the adaptive SIR bias unit, forproviding a SIR estimate adjusted by the adaptive SIR bias unit.
 2. TheWTRU of claim 1 further comprising: a cyclic redundancy check (CRC)error detector configured to detect any errors in the data packets, andgenerate positive acknowledgement (ACK)/negative acknowledgement (NACK)messages which are used to indicate an absence of error and a presenceof error, respectively, in a received data packet.
 3. The WTRU of claim2 wherein the adaptive SIR bias unit comprises: a second summer havingan output electrically coupled to the second input of the first summer;a channel-type correction term generator having an output electricallycoupled to at least one input of the second summer for providingchannel-type correction terms; a SIR prediction term generator having aninput electrically coupled to the SIR estimator and an outputelectrically coupled to another input of the second summer for producingSIR predictive terms; an adaptive correction term generator forproducing an adaptive bias term based upon the ACK/NACK messages inorder to achieve a target block error rate (BLER) of the data packets; abias determination unit, electrically coupled to the adaptive correctionterm generator and the third input of the first summer, for receivingthe adaptive bias term from the adaptive correction term generator andproviding a CQI bias signal to the first summer; and a loop controller,electrically coupled to the bias determination unit and the adaptivecorrection term generator, for controlling the operation of the adaptivecorrection term generator based upon:
 4. An integrated circuit (IC)comprising: a receiver for receiving data packets; asignal-to-interference ratio (SIR) estimator, electrically coupled tothe receiver, for estimating a SIR of the received data packets andproducing a SIR estimate; a first summer having a first inputelectrically coupled to an output of the SIR estimator; an adaptive SIRbias unit, electrically coupled to an output of the receiver and tosecond and third inputs of the first summer, for adjusting the SIRestimate; and a SIR to channel quality indicator (CQI) map unit, havingan input coupled to an output of the first summer, and an output coupledto an input of the adaptive SIR bias unit, for providing a SIR estimateadjusted by the adaptive SIR bias unit.
 5. The IC of claim 4 furthercomprising: a cyclic redundancy check (CRC) error detector configured todetect any errors in the data packets, and generate positiveacknowledgement (ACK)/negative acknowledgement (NACK) messages which areused to indicate an absence of error and a presence of error,respectively, in a received data packet.
 6. The IC of claim 5 whereinthe adaptive SIR bias unit comprises: a second summer having an outputelectrically coupled to the second input of the first summer; achannel-type correction term generator having an output electricallycoupled to at least one input of the second summer for providingchannel-type correction terms; a SIR prediction term generator having aninput electrically coupled to the SIR estimator and an outputelectrically coupled to another input of the second summer for producingSIR predictive terms; an adaptive correction term generator forproducing an adaptive bias term based upon the ACK/NACK messages inorder to achieve a target block error rate (BLER) of the data packets; abias determination unit, electrically coupled to the adaptive correctionterm generator and the third input of the first summer, for receivingthe adaptive bias term from the adaptive correction term generator andproviding a CQI bias signal to the first summer; and a loop controller,electrically coupled to the bias determination unit and the adaptivecorrection term generator, for controlling the operation of the adaptivecorrection term generator based upon:
 7. A wireless transmit/receiveunit (WTRU) comprising: a receiver configured to receive data packets; asignal-to-interference ratio (SIR) estimator configured to estimate aSIR of the received data packets and producing a SIR estimate; anadaptive SIR bias unit configured to adjust the SIR estimate bygenerating correction/predictive terms and a channel quality indicator(CQI) bias signal; and a SIR to CQI map unit electrically coupled to theadaptive SIR bias unit, the SIR to CQI map unit being configured toprovide a SIR estimate adjusted by the adaptive SIR bias unit.
 8. TheWTRU of claim 7 further comprising: a cyclic redundancy check (CRC)error detector configured to detect any errors in the data packets, andgenerate positive acknowledgement (ACK)/negative acknowledgement (NACK)messages which are used to indicate an absence of error and a presenceof error, respectively, in a received data packet.